expect valgrind test failure on memory leak (#15669)

* expect valgrind test failure on memory leak

* alloc 1 instead of 0

testament/tests/shouldfail/tvalgrind.nim

@@ -0,0 +1,17 @@
+discard """
+valgrind: true
+cmd: "nim $target --gc:arc -d:useMalloc $options $file"
+"""
+
+# this is the same check used by testament/specs.nim whether or not valgrind
+# tests are supported
+when defined(linux) and sizeof(int) == 8:
+  # discarding this allocation will cause valgrind to fail (which is what we
+  # want), but valgrind only runs on 64-bit Linux machines...
+  discard alloc(1)
+else:
+  # ...so on all other OS/architectures, simulate any non-zero exit code to
+  # mimic how valgrind would have failed on this test. We cannot use things like
+  # `disabled: "freebsd"` in the Testament configs above or else the tests will
+  # be SKIP-ed rather than FAIL-ed
+  quit(1) # choose 1 to match valgrind's `--error-exit=1`, but could be anything